1. Field of the Invention
This invention is directed to solid electrolytes containing a polymer matrix and an electrolyte solvent (plasticizer) for the polymer matrix. In particular, this invention is directed to solid electrolytes containing an ion salt derivative formed by incorporating an ion salt into the polymer matrix and/or the solvent. The ion salt derivative can partially or completely replace the inorganic ion salt heretofore added to the electrolyte as a separate component in prior art electrolyte compositions.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing a polymer matrix, a solvent and an ion salt derivative incorporated into the polymer matrix and/or the solvent.
This invention is also directed to methods for enhancing the cumulative capacity of the solid electrolytic cells by employing a solid electrolyte which contains an ion salt derivative.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating an inorganic ion salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding their advantages, the manufacture of these solid batteries requires careful process control to minimize the formation of impurities due to decomposition of the inorganic ion salt when forming the solid electrolyte. Excessive levels of impurities inhibit battery performance and can significantly reduce charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable inorganic ion salt as a separate component. The inorganic matrix may be non-polymeric [e.g, .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which, prior to the present invention, has been added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
Regardless of which of the above techniques is used in preparing the solid electrolyte, a recurring problem has been the presence of impurities which interfere with cell function and can reduce battery life. The source of these impurities is the partial decomposition of the inorganic ion salt formed in the polymer matrix. Partial decomposition occurs due to exposure of the inorganic ion salts to the high temperatures used, for example, in forming the polymer matrix and/or in evaporating the volatile solvent. These high temperatures cause the salt to break down into insoluble or less soluble salts. For example, lithium hexafluorophosphate (LiPF.sub.6) is converted to LiF, which is much less soluble in the electrolyte and can precipitate out. Such insoluble or less soluble salts cannot function to transfer electrons, and hence the resulting battery is rendered less efficient.
Thus, in preparing the solid electrolyte, great care must be taken to maintain processing temperatures below the threshold level for significant salt decomposition. The need for careful monitoring of process temperatures increases manufacturing costs and at the same time results in a percentage of the solid electrolyte produced being off specification due to unavoidable process temperature variation. Electrolyte material meeting production specifications generally contains small but tolerable levels of impurities which can nevertheless affect cell performance, particularly with respect to cumulative capacity. Cumulative capacity of a solid battery is defined as the summation of the capacity of the battery over each cycle (charge and discharge) in a specified cycle life.
Quite apart from the problem of decomposition is the cost of the inorganic ion salts. Simple salts such as lithium halides are less preferred in the electrolyte because they are not very compatible with the polymers used in forming the matrix (and hence can precipitate out as mentioned above). More complex salts are favored because of their greater compatibility, but are more costly. A highly preferred salt is LiPF.sub.6, but this salt has been found to be very heat sensitive and quite expensive. Another preferred salt is lithium hexafluoroarsenate (LiAsF.sub.6). This salt poses significant disposal problems due to the presence of arsenic.
Notwithstanding their complexity and costs, even under the best of circumstances (e.g. impurity levels approaching zero), the inorganic ion salts typically have a transference number between 0.4 and 0.55, meaning that the ion salt carries only between 40% and 55% of the total plus (+) charge.
In view of the above, the art is searching for methods to reduce impurities in solid electrolyte manufacture as well as to increase the cumulative capacity of solid batteries employing such electrolytes.